The present invention relates to a novel dry writing-board and a process for producing the same.
A writing-board which has been widely used is a so-called "black board" comprising a wood panel coated with a black or green paint. In writing on the black board, a white or other colored chalk is used. When the black board is erased or wiped, the chalk is powdered and scattered. The powdered chalk not only adheres to the body of the user and his clothes, but it is also inhaled into his body. Therefore, the use of chalk is undesirable for sanitary reasons.
In contrast, there has been developed a so-called "white board" comprising a metal plate covered with any of various white plastic paints. The white board is ordinarily written on with a liquid black or other colored marker. When the inscriptions on the board are to be erased, they are rubbed with a dry cloth or paper. In this case, writing and erasing are not accompanied by any scattering of powder. Therefore, the white board can eliminate the disadvantages encountered in the use of the black board.
However, when a white board having thereon a plastic coating is used for writing, the inscriptions written by a marker quite frequently cannot be completely erased with a dry cloth or paper, and the dye contained in the marker inevitably remains on the surface of the board.
As an attempt to overcome this difficulty, a white board comprising a metal plate coated with a glaze instead of plastics has been proposed. In the case of this white board, the inscriptions written by a marker can be completely erased with a dry cloth or paper.
Such a dry writing board called a white board has been ordinarily prepared by putting on the ground coat of a metal plate a glaze of a composition which is 50 to 55% of silicon oxide, 23 to 30% of titanium oxide, 5 to 6% of sodium oxide, 3 to 6% of potassium oxide and 5.5 to 12.5% of boron trioxide, all percentages being by weight. The glaze is sprayed on the ground coat and the coated plate is fired in a furnace at a temperature of about 800.degree. C.
In the production of such a writing board, it is generally necessary to whiten and vitrify the surface of the board and to increase the acid resistance (anti-rust effect) and covering power (impermeability to the color of the ground coat) of the board. Further, it is necessary that the control of the production process, i.e., firing temperature, be effected with ease. This is because the ground coat or glaze must be fired at a temperature below the A.sub.3 transformation point of steel used as a substrate, preferably a temperature of not greater than 850.degree. C. For this reason, the glaze composition contains a greater proportion of titanium oxide and silicon oxide as stated above. However, when the proportion of titanium oxide in the glaze composition exceeds about 29% by weight, the gloss of the surface of the board reaches about 95% which makes it completely impossible for observers to read the inscriptions written on the board at some angles. The gloss of the board is determined under conditions such that both the incidence angle and reflection angle of light are 60.degree. and the gloss of a standard black glass is 100%. If the gloss is not greater than about 65%, the inscriptions written on the board can be easily read.
Heretofore, the halation due to an excessively high surface gloss has been prevented by setting the writing board at a position easy to see by means of a supporting member or inclining the board itself at some angle. In this case, an apparatus for controlling the angle of inclination is required, and the entire writing board becomes large-scaled. Such a large-scaled writing board is also complicated to operate.